Cargo boxes typically used on tractor trailers are provided with a pair of hinged rear doors which swing outwardly to provide access to the cargo box. When the doors are swung fully open the access opening is virtually equal to the box cross-section. After the cargo has been loaded the doors are swung to the closed position and each is latched shut by imparting rotation to a vertical latch bar by means of a handle to cause upper and lower latch jaws to lock in receptacles fixed in the door frame. After the doors are closed, no opening exists whereby the state of the cargo may be inspected. After a highway trip, it is not uncommon for the cargo to shift and the contents of the box may come loose and slide or roll against one or both doors. When the latches holding a door closed are released, the door may be forcibly driven outwardly by the items pressing against it. The forceful action of the door and any items falling from the trailer as the door opens poses a great risk of serious injury to persons in the vicinity of the trailer doors.
While devices are known for restraining the opening of a cargo box door, some of these are mechanically complicated and require modification or retrofitting of the latch mechanism and, in general, are limited to use on the trailer to which the devices have been fitted. Other devices of more simple design are known but these also cannot be used interchangeably for many types of trailers.